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Bad blood?

6 min read December 26, 2015 at 11:51am on Health and Infection

As regular as clockwork, at this time of year, articles appear about the Australian Red Cross Blood Service and their desperate need for blood donations across the holiday period.  If you're eligible, please consider giving.  

If you move in the circles I do, then shortly after this comes the call to open donations up for men who have sex with men.   I saw this excellent article by Nic Holas (twitter) in the Sydney Morning Herald just before Christmas, talking about the issue of whether or not MSM should be able to give blood.  It's a controversial topic - and I think Nic's article was excellent.

It's something I've discussed a few times at work, and have often meant to blog about, so thanks to Nic for giving me the kick up the bum to actually do it.  

Warning:  Post contains medical-school style evidence-based medicine, 2x2 tables, generous use of Fermi estimations, and all my personal opinions which are nothing to do with anyone I work with or for and are not medical advice etc etc.

So.  I'm going to leave aside the ethical question of whether or not gay men should be able to donate, and just focus on the risk-management and the maths.  

But first, for those of you that haven't done med school EBM, some definitions:

Sensitivity (of a test):  The likelihood a test will correctly detect the thing that you're testing for (ie: in this context, how likely is a HIV test to be positive, assuming the donor has HIV. Another way of expressing it is 1-(false negative rate) )

Specificity (of a test): The likelihood a positive result from a test truly represents a positive result (ie:  if the test is positive, does the donor really have HIV - or another way, it's 1-(false positive rate of the test))

Sensitivity and specificity are functions of the test being used.  Obviously, you'd like a test where these are very close to 100%, particularly in the setting of HIV testing, but in reality, no test is perfect, and you often end up with a trade-off - highly sensitive tests (ie: few false negatives) may come at the expense of a slightly lower specificity (ie: a few false positives).

Positive predictive value (PPV):  What is the chance that a positive test result is actually a truly HIV+ person rather than a false positive?

Negative predictive value (NPV):  What is the chance that a negative test result is a truly HIV- person rather than a false negative.

Unlike the sensitivity and specificity, PPV and NPV depend on both the test, but also the disease prevalence - which is a key issue for this discussion, and I'll illustrate by way of example.  

The key to understanding the blood donor screening questionnaire is to think of it as a risk-mitigation tool.  The Red Cross Blood service want to manipulate the population of donors such that the risk of HIV-positivity is as low as possible;  this in turn makes the likelihood of a false negative test even smaller.  

As Nic points out in his article, HIV disproportionately affects men who have sex with men, therefore by excluding them, the donor pool is less likely to have positive individuals within it. From the ASHM PEP guideline (pdf), HIV seroprevalence various populations are as follows:

  • MSM in major Australian centres:  4-12%
  • MSM who inject drugs: 29%
  • Non-MSM PwID: 1%
  • Whole Australian population: 0.1%
  • People who "pass" the blood donor questions:  0.0004%

Lets call the prevalence in MSM a nice round 10% (because it makes my numbers easier), and round the blood donor prevalence up to 0.001% for the same reason (this will reduce the difference in risk somewhat).

Modern HIV tests are pretty good - quoted performance is a sensitivity (picking up people with HIV) of 99.8% and a specificity (positive test is correct) of 98%. So, you can construct a cross-table of the prevalence of the virus and the performance of the test, and I've done this under three different scenarios:

  • people who are eligible to donate based on the ARCBS questionnaire;
  • the unscreened general population;
  • a population with HIV seroprevalence of 10%

Lets see what it shows: 

(tables not supported in the new software I use for my blog - old post here)

So, you can see from these tables, that the screening questions improve the performance of the test by reducing the pre-test probability of HIV infection in the population;  it is essentially a way of minimising the small risk of a false negative test to becoming an even more rare event.  

Hopefully this example has shown that there is benefit in donor screening. The question is then, is it reasonable for any sex with a man being reason for deferral? Seeing a sex worker in the previous 12 months is also indication for deferral, despite the fact that it is a requirement that Australian sex workers are tested every three months and also that sex workers do not offer unprotected services. As well as this, just as HIV is unevenly distributed in the general population, it's also unevenly distributed amongst gay men - older men who may have been diagnosed in the 80s, sexually adventurous men (known in the sexual health business as SAMs).  

At what point do you decide to jump off the curve of diminishing returns in making the donor questions even more complex?

It's particularly worth thinking about this in light of the final estimation I'll do in this post.

Around 3.5% of the eligible donor population give blood. Given there are 25,000 people living with HIV in Australia, and around 75% acquired it through MSM, lets say that there are 20,000 MSM with HIV.  Based on a seroprevalence of 10%, that means there are around 200,000 MSM in Australia.  Say 5% of them would give blood if they were eligible, that would be an extra 10,000 donors - or an increase in the donor pool by 2%.

Sure, a 2% increase in the number of donors would give some extra reserve in the system, but would need either a major overhaul of the donor screening questions and the increased potential for false negative tests. ARCBS have reviewed the guidelines relating to sexual behaviours and deferrals in 2012, and based on the principle that blood safety was paramount, elected to leave restrictions as they are.  The full report (pdf) acknowledges that this policy may exclude some low-risk MSM, but on the balance felt that removing the restriction was an unacceptable risk.  

There's no easy answer to this question, and to be honest, I'm not entirely decided either way.

(thanks to Nic Holas for his article and to Daniel Reeders, -his blog name I have shamelessly ripped off for the title of this post).

Featured image: Blood donation - Image by ANKAW√ú via Wikimedia Commons - CC-BY-SA-3.0